1. Field of the Invention
The invention relates to the field of non-thermal plasma. In particular the invention relates to methods and apparatus for application of non-thermal plasma.
2. Description of the Related Technology
Plasma is often described as the fourth state of matter. Typically it contains charged electrons and ions as well as chemically active species such as ozone, hydroxyl radicals, nitrous oxides, electronically excited atoms and molecules. Electronic excitation of some atoms and molecules in plasma produces ultraviolet radiation (hereinafter “UV”). Plasma can also be a good electrical conductor due to the presence of charged particles in the plasma. In a room-temperature environment, plasma is usually supported by electro-magnetic fields. Light electrons absorb energy from an electric field and transfer part of this energy to heavy particles in the plasma. Plasma is considered to be thermal if the rate of the electron energy transfer is fast relative to the rate of energy losses by heavy particles. In this case heavy particles reach energies comparable with the energy of electrons and the plasma becomes hot. In other cases, when electrons are not given sufficient opportunity to transfer their energy, heavier plasma components remain at much lower temperatures than the electrons. Such plasmas are called non-thermal and their gas temperatures can be as low as room temperature.
Plasma resulting from electric discharges has been employed in the past for cauterization which primarily involves transfer of thermal energy to tissue. An example of such treatment is a treatment which uses the Argon Plasma Coagulator™ (hereinafter “APC”) and related types of equipment. These devices create plasma in a flowing gas (such as argon) using a radio frequency (hereinafter “RF”) electromagnetic field. Plasma in these devices plays the role of a soft electrode which is used to transfer substantial current (usually greater than 150 milli-Amperes and possibly exceeding 1 Ampere for short periods of time) into the tissue. This results in rapid heating of the tissue to over 100° C., typically causing tissue desiccation and damage. FIGS. 1a-1e show examples of damage that may be caused by use of thermal plasma for tissue treatment. FIG. 1a shows tissue overheating caused by thermal plasma. FIG. 1b shows puncturing of the skin tissue as a result of contact with thermal plasma. FIG. 1c shows schar formation on the skin tissue that may be caused by thermal plasma. It should be mentioned that, in more conventional electro-cautery devices, conducting electrodes made of solid materials are employed to transfer currents that heat the tissue. Tissue can stick to the solid electrodes upon heating and the use of plasma in place of a solid electrode circumvents this problem in APC, for example.
Thermal plasma devices that do not rely on delivery of current into tissue have also been developed for coagulation and cauterization of tissue. Instead, the plasma is employed to rapidly heat a gas. The heated gas (often argon due to its inert properties) is subsequently directed toward the tissue in the form of a jet whereby the heated gas transfers its thermal energy to the tissue. Examples of devices for implementation of this type of technology are the PlasmaJet™ distributed by Plasma Surgical Limited and systems patented by Rhytech Corporation (U.S. Pat. Nos. 6,629,974 and 6,723,091, and U.S. Published patent application no. US2006/0009763). The effect of such plasma treatment is mostly thermal because many of the active chemical species in the remotely created plasma are short-lived and do not survive transport of the heated gas flow to the tissue.
Thus, it is well known than electrical discharge plasma has a very strong influence on living tissue. This strong influence can be of two kinds: thermal and non-thermal. Thermal influence of plasma which results in rapid heating of living tissue is well studied and is used for, for example, cauterization. In other cases the thermal influence of plasma results in living tissue desiccation and burns and thus is undesirable.
The non-thermal influence of electrical discharge plasma, caused by active plasma particles (electrons, ions, radicals, and other chemically active species) and UV radiation, may be useful in many cases, for example, for living tissue disinfection and sterilization, for skin disease treatment, for blood coagulation, etc. The closer to the living tissue the active plasma is located and the higher is electrical field in the plasma, the higher the intensity and efficacy of the non-thermal plasma treatment. Available methods of non-thermal plasma treatment are relatively weak and are effected usually by plasma jet or afterglow treatment because there are limitations on the power flux to the living tissue (to prevent overheating of the tissue) and on the total current and current density which may flow through the living tissue (to prevent damage of the tissue and nerve channels). Since the power of electrical discharge that creates plasma is a product of the discharge current and voltage, the higher the voltage—the lower the current, when power is fixed.
To increase efficacy of non-thermal plasma treatment and to overcome existing limitations, the present invention employs tissue as an electrode of a high-voltage electrical discharge with relatively low total current and current density. Under these conditions, the highest concentration of active plasma factors are located in close proximity to the treated living tissue, while the temperature of the plasma remains low because of the use of a relatively low total discharge power. In addition, total current and current density will also be low to ensure that tissue and nerve channels are not damaged.
Non-thermal plasmas have been developed. Non-thermal plasma discharges are used for the sterilization of equipment and various implantable plastics, for biochemical surface functionalization and treatment, and for many other applications. However, as far as the inventors are aware, non-thermal plasma technology has not been used for the various medical applications described herein, where plasma is in direct electrical contact with living tissue and acts on living tissue through various plasma-chemical processes, rather than primarily by transfer of thermal energy.
Therefore, there exists a need for providing a method for living tissue treatment by plasma without causing thermal damage.